This invention relates to an automatic FM tuner of a voltage synthesizer type and more particularly to an automatic FM tuner of a voltage synthesizer type having an improved detecting of the tuning point.
An automatic FM tuner of a voltage synthesizer type is, as shown in FIG. 1 (a), comprised of an antenna 1, a high frequency unit 2, a mixing circuit 3, an intermediate frequency/FM detector circuit 4, a voltage control oscillator 5, and a voltage synthesizer controller 6 for setting an output frequency of the voltage control oscillator 5 by D/A transformation of the tuning clock pulse. In an automatic FM tuner of a voltage synthesizer type, a detecting of the tuned frequency is performed such that the output voltage with S-shaped wave form of FM detector circuit 4 is fed to a window comparator 7 and the output from the window comparator 7 is applied as a stop signal during a search of the broadcasting station. For performing this operation, a conventional or prior art system performed in such a way as when the frequency was searched or scanned toward an increased broadcasting frequency as shown in FIG. 1 (b), the tuning clock pulses shown at B in FIG. 1 (b) are counted from the decreasing point of the output S.sub.1 of the window comparator 7 having as its input the output voltage with S-shaped wave form of FM detector circuit 4 shown at a curve (A) in FIG. 1 (b). To the contrary, when the frequency was searched or scanned toward a decreased broadcasting frequency, the tuning clock pulse was counted from a decreasing point of the output S.sub.1 of the window comparator 7 and then a position of the desired counting value was applied as a tuning point.
However, a width of the stop signal ( a width between the outputs S.sub.1 and S.sub.2 of the window comparator 7) and the number of tuning clock pulses in the width for every band of the tuner are different and even in a band, they are different in response to the broadcasting frequency. This is due to the fact that the tuning clock pulse has a fixed 10 to 20-bits and that a relation between an oscillation frequency of the voltage control oscillator 5 and a tuning clock pulse is not made to be linear.
Therefore, there was such a problem as the tuning operation was performed by an automatic tuning operation after the broadcasting station was detected during a search of the broadcasting station, and even if a tuning was matched, it was not necessarily to have a correct tuning at the most suitable tuning point.